To perceive the status of their light environment, plants have evolved various photoreceptors. The photoreceptors (an unidentified UV-B receptor, a phototropin and cryptochrome sensing TV-A/blue light region of the spectrum, and phytochromes sensing red (R)/far-red (FR) region of the spectrum) mediate signals to genes regulating the growth and development of plants (Fankhauser, C. & Chory, J. Curr. Biol., 9:R123-R126, 1999; Neff, M. M., et al., Genes Dev., 14:257-271, 2000). The recent development of molecular biological and biochemical research technologies and molecular genetic research technologies have been made in the molecular cloning and genetic characterization of the photoreceptors themselves, as well as some signal intermediate components involved in transducing perceived signals from photoreceptors to photoresponsive genes (Quail, P. H. Curr. Opin. Cell. Biol., 14:180-188, 2002; Gyula, P. et al., Curr. Opin. Plant Biol., 6:446-452, 2003).
Phytochromes are photoreceptors whose characteristics were most well studied, that regulate various aspects of the growth and development of higher plants. Depending to the spectrum of light irradiated on phytochromes, a reversible photo-conversion occurs between a biologically inactive, red light-absorbing form (Pr) and a biologically active, far-red light-absorbing form (Pfr). The photo-conversion into the Pfr form by red light treatment initiates translocation from the cytoplasm of phytochromes themselves to the nucleus, and activates the signal transduction pathway inducing various effects on the expression and development of genes, thus regulating the growth and development of plants (Quail, P. H. Curr. Opin. Cell. Biol., 14:180-188, 2002; Fankhauser, C. & Chory, J. Curr. Biol., 9:R123-R126, 1999). It is reported that there are five different phytochromes (designated phyA, phyB, phyC, phyD and phyE) in Arabidopsis thaliana (Neff, M. M., et al., Genes Dev., 14:257-271, 2000; Quail, P. H. Curr. Opin. Cell. Biol., 14:180-188, 2002).
The generic phytochromes consist of an apoprotein of about 116-121 kDa and a tetrapyrrole chromophore, phytochromobillin that is covalently linked to the apoprotein (Quail, P. H. Curr. Opin. Cell. Biol. 14:180-188, 2002; Gyula, P. et al., Curr. Opin. Plant Biol., 6:446-452, 2003). The photosensoty activity of phytochromes resides in their unique capacity for reversible, light-induced interconversion between the Pr form and the Pfr form. The monomer of phytochrome molecule is composed of a globular N-terminal domain (˜70 kDa), which is anchoring the chromophore, and a C-terminal domain linked via a flexible hinge region. The N-terminal domain is responsible for photosensory function. Also, the conformationally open C-terminal domain (about 55 kDa) is known to be involved in signal transfer (Quail, P. H. Curr. Opin. Cell. Biol., 14:180-188, 2002; Gyula, P. et al., Curr. Opin. Plant Biol., 6:446-452, 2003). The C-terminal domain contains a pair of the Per-Arnt-Sim (PAS) motifs around the regulatory core region. The PAS motifs are known to be involved in protein-protein interaction and inter-domain communications in some sensory proteins. The results of analysis with recombinant oat phytochrome A showed that the C-terminal domain of phytochromes possesses serine/threonine protein kinase activity (Yeh, K. C. & Lagarias, J. C. Proc. Natl. Acad. Sci. U.S.A. 95:13976-13981, 1998; Fankhauser, C. et al., Science, 284:1539-1541, 1999). Furthermore, it was suggested by the results of spectral and photochemical tests that the photo-isomerization of phytochromes induced by chromopores triggers conformational changes throughout the whole phytochrome molecule via inter-domain communication within the molecules, like the well-characterized rhodopsin visual receptor in animals (Maeda, T. et al., Prog. Retin. Eye Res., 22:417-434, 2003; Vishnivetskiy, S. A. et al., J. Biol. Chem., 275:41049-41057, 2000). In addition, the conformational signals of phytochrome could be further differentiated by inter-domain interactions in the phytochrome molecule, and this is presumed to be modulated by reversible phosphorylation/dephosphorylation at serine residue in the hinge region. In spite of these many authentic findings, however, minute mechanisms by which the phytochromes transduce light signals to photoresponsive genes are not yet completely established.